A typical vane-type camshaft phaser for changing the phase relationship between a crankshaft and a camshaft of an internal combustion engine generally comprises a plurality of outwardly-extending vanes on a rotor interspersed with a plurality of inwardly-extending lobes on a stator, forming alternating advance and retard chambers between the vanes and lobes. Engine oil is selectively supplied to one of the advance and retard chambers and vacated from the other of the advance and retard chambers by a phasing oil control valve in order to rotate the rotor within the stator and thereby change the phase relationship between the camshaft and the crankshaft. Some camshaft phasers incorporate a valve spool within a camshaft phaser attachment bolt which is used to secure the camshaft phaser to the camshaft. The valve spool is moved axially within the camshaft phaser attachment bolt to open and close passages which results in oil being directed to and from the advance and retard chambers as needed in order to rotate the rotor within the stator. The clearance between the valve spool and the camshaft phaser attachment bolt must be minimized in order to control leakage between the corresponding interface; however, the clearance between the valve spool and the camshaft phaser attachment bolt must be sufficiently great to accommodate radially inward expansion of the camshaft phaser attachment bolt when the camshaft phaser attachment bolt is tightened to the camshaft in order to avoid binding of the valve spool within the camshaft phaser attachment bolt. The camshaft phaser attachment bolt may typically expand radially inward by 0.010 mm diametrically with great variability. Consequently, in order accommodate radially inward expansion of camshaft phaser attachment bolt and variability thereof, a larger clearance than desired to minimize leakage may need to be provided in order to ensure proper movement of the valve spool after the camshaft phaser attachment bolt is tightened to the camshaft.
What is needed is camshaft phaser which minimizes or eliminates one or more the shortcomings as set forth above.